A conventionally known ignition device contains an ignition coil including a primary coil and a secondary coil, and an ignition plug connected to the secondary coil. The energization of the primary coil is turned on/off to cause electromagnetic induction to supply energy to the ignition plug, and thereby spark discharge is generated. Further, as a means for continuing a spark discharge once generated, there is known an ignition device as follows including first and second circuits.
The first circuit is a conventionally well-known ignition circuit for starting spark discharge at an ignition plug by turning on/off the energization of the primary coil. For example, the first circuit is configured by connecting the (+) pole of the battery with the (+) terminal of the primary coil, the (−) terminal of the primary coil with the ground, and placing a switch for starting discharge on the (−) side of the primary coil.
Further, the second circuit is for continuing the spark discharge started by the operation of the first circuit, and during the spark discharge started by the operation of the first circuit, it energizes the primary coil in a direction opposite to that of the first circuit. The second circuit maintains the energization of the secondary coil in the same direction as that initiated in the operation of the first circuit to continue supplying energy to the ignition plug, so that the spark discharge continues. Further, for example, the second circuit is configured by being connected between the primary coil and the ignition switch with respect to the first circuit, and providing a switch for turning on/off the power supply from the boost circuit to the primary coil (see, for example, PTL 1).
It is stated that if the electric energy of the boost circuit is supplied from the (−) side of the primary coil by turning on/off the second switch to continue the spark discharge in such configuration, it is possible to continue spark discharge while reducing the burden on the ignition plug and also reducing unnecessary power consumption.
Meanwhile, when a battery is used as the destination of the return flow of the current supplied to the primary coil by the operation of the second circuit, a problem as follows may arise that overvoltage may be applied to other devices. That is, when a battery is adopted as the destination of the return flow, a return circuit for allowing the current supplied to the primary coil to flow to the battery is provided in the ignition device together with the first and second circuits. In such an ignition device, when the so-called load dump state, a state where the connection between the return circuit and the battery is broken due to bad connection, detachment of the battery terminal, or the like, occurs, if the supply of energy by the second circuit is continued, the voltage of the return circuit rises and overvoltage may be applied to other devices connected to the return circuit.
PTL 2 discloses the following configuration in relation to an ignition device for an internal combustion engine having a first circuit and a boost circuit. That is, according to the ignition device of PTL 2, the waveform of the voltage supplied from the battery to the ignition device is monitored, and whether there is an abnormality in the regulator of the alternator or the battery is determined based on this waveform. When it is determined that there is an abnormality, the power supply to the ignition device is stopped, and when it is determined that there is no abnormality, the power supply to the ignition device is resumed.
Thus, according to the ignition device of PTL 2, the ignition device is protected from the overvoltage during the period from occurrence of the abnormality to return to the normal voltage, and when the voltage has returned normal, the ignition device can immediately operate normally without manual recovery work or the like on the control. In addition, according to such configuration, even if a load dump state occurs, the ignition device can be protected from overvoltage.
However, in a case where the ignition device includes a second circuit and the destination of the return flow is a battery, as a countermeasure against the occurrence of the load dump state, it is necessary to consider protection of other devices besides protection of the ignition device itself. The ignition device of PTL 2 does not consider protection of other devices.